From: Vincent Mailhol > Sent: 02 December 2024 17:33 > > From: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx> > > __is_constexpr(), while being one of the most glorious one liner hack > ever witnessed by mankind, is overly complex. Following the adoption > of C11 in the kernel, this macro can be simplified through the use of > a _Generic() selection. You should give credit to some of the earlier patches that do the same. I'm sure there were some related ones from Linus - not applied yet. > > First, split the macro in two: > > - __is_const_zero(x): an helper macro; tells whether x is the > integer constant expression 0 or something else. > > - is_const(x): replacement of __is_constexpr(); tells whether x is a > integer constant expression. > > The split serves two purposes: first make it easier to understand; > second, __is_const_zero() will be reused as a building block for other > is_const_*() macros that will be introduced later on. > > The core principle of __is_constexpr() to abuse the return type of the > ternary operator remains, but all the surrounding sizeof() hack > disappear. > > On a side note, while not relevant to the kernel, __is_constexpr() > relied on the GNU extension that sizeof(void) is 1. const_expr() does > not use any GNU extensions, making it ISO C compliant. > > __is_constexpr() is temporarily kept and will be removed once all its > users get migrated to is_const() (or one of its friend). > > Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx> > --- > include/linux/compiler.h | 41 +++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 41 insertions(+) > > diff --git a/include/linux/compiler.h b/include/linux/compiler.h > index a2a56a50dd85227a4fdc62236a2710ca37c5ba52..30ce06df4153cfdc0fad9bc7bffab9097f8b0450 100644 > --- a/include/linux/compiler.h > +++ b/include/linux/compiler.h > @@ -316,6 +316,47 @@ static inline void *offset_to_ptr(const int *off) > #define statically_true(x) (__builtin_constant_p(x) && (x)) > #define statically_false(x) (__builtin_constant_p(x) && (x) == 0) > > +/* > + * Whether x is the integer constant expression 0 or something else. > + * > + * Details: > + * - The C11 standard defines in §6.3.2.3.3 > + * (void *)<integer constant expression with the value 0> > + * as a null pointer constant (c.f. the NULL macro). > + * - If x evaluates to the integer constant expression 0, > + * (void *)(x) > + * is a null pointer constant. Else, it is a void * expression. > + * - In a ternary expression: > + * condition ? operand1 : operand2 > + * if one of the two operands is of type void * and the other one > + * some other pointer type, the C11 standard defines in §6.5.15.6 > + * the resulting type as below: > + * if one operand is a null pointer constant, the result has the > + * type of the other operand; otherwise [...] the result type is > + * a pointer to an appropriately qualified version of void. > + * - As such, in > + * 0 ? (void *)(x) : (char *)0 > + * if x is the integer constant expression 0, operand1 is a null > + * pointer constant and the resulting type is that of operand2: > + * char *. If x is anything else, the type is void *. > + * - The (long) cast silences a compiler warning for when x is not 0. > + * - Finally, the _Generic() dispatches the resulting type into a > + * Boolean. The comment is absolutely excessive. I'm sure I managed about 2 lines in one of the patches I did. > + * > + * Glory to Martin Uecker <Martin.Uecker@xxxxxxxxxxxxxxxxxxxxx> IIRC Martin has agreed in the past that the accreditation can be removed - especially since it refers to the 'sizeof (void)' trick. > + */ > +#define __is_const_zero(x) \ > + _Generic(0 ? (void *)(long)(x) : (char *)0, char *: 1, void *: 0) > + > +/* > + * Returns a constant expression while determining if its argument is a > + * constant expression, most importantly without evaluating the argument. You need to differentiate between a 'constant integer expression' and a 'compile time constant'. > + * > + * If getting a constant expression is not relevant to you, use the more > + * powerful __builtin_constant_p() instead. __builtin_constant_p() is not 'more powerful' it is testing for something different. David > + */ > +#define is_const(x) __is_const_zero(0 * (x)) > + > /* > * This is needed in functions which generate the stack canary, see > * arch/x86/kernel/smpboot.c::start_secondary() for an example. > > -- > 2.45.2 > - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
